Further studies on the roles of sodium and potassium in the generation of the electro-olfactogram. Effects of mono- , di- , and trivalent cations

Enhancement of olfaction by femtomolar concentrations of zinc ions

Zinc is recognized as an important element for olfaction. Zinc nanoparticles enhance olfaction in response to odors; however, the mechanisms underlying this action remain unknown. Herein, the effect of zinc on olfactory receptors was deduced using electro-olfactogram (EOG) responses recorded from the isolated olfactory mucosae of bullfrogs (Rana catesbeiana) following the administration or chelation of zinc ions. Menthone and n-amyl acetate were used as odorants, whereas forskolin (an adenylate cyclase activator) and cholera toxin (a Gαolf activator) were used as intracellular signal transduction activators. The EOG responses provoked by the odorants and cholera toxin were suppressed by dithizone-mediated zinc ion chelation, and the EOG responses were recovered by administering non-chelated zinc. However, the EOG response to forskolin was not suppressed by dithizone. In contrast, the addition of femtomolar concentrations of zinc ions enhanced the EOG responses. The above-mentioned effects on EOG responses were examined by changing the concentration of zinc ions but not zinc nanoparticles. The results of this study suggest that Gαolf alone or both olfactory receptors and Gαolf likely require zinc ions for their activation.

Effect of Deepwater Horizon Crude Oil Water Accommodated Fraction on Olfactory Function in the Atlantic Stingray, Hypanus sabinus

The Deepwater Horizon oil spill was the largest accidental marine oil spill in history, releasing nearly 5 million barrels of crude oil. Crude oil causes both lethal and sublethal effects on marine organisms, and sensory systems have the potential to be strongly affected. Marine fishes rely upon the effective functioning of their sensory systems for detection of prey, mates, and predators. However, despite the obvious importance of sensory systems, the impact of crude oil exposure upon sensory function remains largely unexplored. Here we show that olfactory organ responses to amino acids are significantly depressed in oil exposed stingrays. We found that the response magnitude of the electro-olfactogram (EOG) to 1 mM amino acids decreased by an average of 45.8% after 48 h of exposure to an oil concentration replicating that measured in coastal areas. Additionally, in oil exposed individuals, the EOG response onset was significantly slower, and the clearing time was protracted. This study is the first to employ an electrophysiological assay to demonstrate crude oil impairment of the olfactory system in a marine fish. We show that stingrays inhabiting an area impacted by an oil spill experience reduced olfactory function, which would detrimentally impact fitness, could lead to premature death, and could cause additional cascading effects through lower trophic levels.

Recording odor-evoked response potentials at the human olfactory epithelium

Electro-olfactogram (EOG) represents the sum of generator potentials of olfactory receptor neurons in response to an olfactory stimulus. Although this measurement technique has been used extensively in animal research, its use in human olfaction research has been relatively limited. To understand the promises and limitations of this technique, this review provides an overview of the olfactory epithelium structure and function, and summarizes EOG characteristics and conventions. It describes methodological pitfalls and their possible solutions, artifacts, and questions of debate in the field. In summary, EOG measurements provide a rare opportunity of recording neuronal input from the peripheral olfactory system, while simultaneously obtaining psychophysical responses in awake humans.

Effects of Ca2+ and calmodulin on adenylyl cyclase activity in sheep olfactory epithelium

Sheep olfactory epithelium contains an adenylyl cyclase which is stimulated by many but not all odorants. Here we report that this enzyme is activated by calmodulin in a dose-dependent manner, and that calcium ions are required for this response. Odorant stimulation of adenylyl cyclase is unaffected by the complex Ca2+/calmodulin, as suggested by the results obtained both in Ca2+/calmodulin-depleted membranes and under calmodulin antagonist treatment; this confirms the prediction that the Ca2+ binding protein and odorants stimulate the olfactory adenylyl cyclase through parallel mechanisms. The persistent activation of the regulatory component of adenylyl cyclase by GppNHp does not alter the response of the enzyme to either odorant or Ca2+/calmodulin. In sheep olfactory epithelium a cAMP-phosphodiesterase activity is also present, which is highly inhibited by IBMX and aminophylline, scarcely by RO 20-1724, and unaffected by Ca2+/calmodulin. The modulatory role exerted by calcium on cAMP system in sheep olfactory signal transduction is discussed.

Current recording from sensory cilia of olfactory receptor cells in situ. II. Role of mucosal Na+, K+, and Ca2+ ions

Action potential-driven current transients were recorded from sensory cilia and used to monitor the spike frequency generated by olfactory receptor neurons, which were maintained in their natural position in the sensory epithelium. Both basal and messenger-induced activities, as elicited with forskolin or cyclic nucleotides, were dependent on the presence of mucosal Na+. The spike rate decreased to approximately 20% when mucosal Na+ was lowered from 120 to 60 mM (replaced by N-methyl-D-glucamine+), without clear changes in amplitude and duration of the recorded action potential-driven transients. Mucosal Ca2+ and Mg2+ blocked spike discharge completely when increased from 1 to 10 mM in Ringer solution. Lowering mucosal Ca2+ below 1 mM increased the spike rate. These results can be explained by the presence of a cyclic nucleotide-dependent, Ca(2+)-sensitive cation conductance, which allows a depolarizing Na+ inward current to flow through the apical membrane of in situ receptor cells. A conductance with these properties, thought to provide the receptor current, was first described for isolated olfactory cells by Nakamura and Gold (1987. Nature (Lond.). 325:442-444). The forskolin-stimulated spike rate decreased when l-cis-diltiazem, a known blocker of the cyclic nucleotide-dependent receptor current, was added to the mucosal solution. Spike rate also decreased when the mucosal K+ concentration was lowered. Mucosal Ba2+ and 4-aminopyridine, presumably by means of cell depolarization, rapidly increased the spike rate. This suggests the presence of apical K+ channels that render the receptor cells sensitive to the K+ concentration of the olfactory mucus. With a slower time course, mucosal Ba2+ and 4-aminopyridine decreased the amplitude and caused rectification of the fast current transients (prolongation of action potentials). Abolishment of the apical Na+ current (by removal of mucosal Na+), as indicated by a strong decrease in spike rate, could be counteracted by adding 10 mM Ba2+ or 1 mM 4-aminopyridine to the mucosal solution, which re-established spiking. Similarly, blockage of the apical cation conductance with 10 mM Ca could be counteracted by adding 10 mM Ba2+ or by raising the mucosal K+ concentration. Thus mucosal concentrations of Na+, K+, and Ca2+ will jointly affect the sensitivity of odor detection.

Molecular physiology of olfaction

Olfactory reception is mediated by olfactory receptor cells located in the olfactory epithelium. These cells are bipolar neurons that extend a dendrite toward the nasal lumen and an axon toward the olfactory bulb in the brain. The dendrite possesses a group of apical cilia embedded in mucus. Odorant recognition and signal transduction are initiated at the membranes of these chemosensory cilia and culminate in excitation of the olfactory receptor cell. Differential activation by odorants of distinct groups of olfactory receptor cells generates patterns of neuronal activity that encode odor quality and concentration. The identities of primary odorant recognition sites at the ciliary membrane remain to be established. However, a significant body of information has become available with respect to olfactory transduction mechanisms. It is now becoming clear that olfactory transduction involves the interplay of several second messenger systems to control the responses of these exquisitely sensitive chemosensory neurons.

Odorant response of isolated olfactory receptor cells is blocked by amiloride

Olfactory receptor cells were isolated from the nasal mucosa of Rana esculenta and patch clamped. Best results were obtained with free-floating cells showing ciliary movement. 1) On-cell mode: Current records were obtained for up to 50 min. Under control conditions they showed only occasional action potentials. The odorants cineole, amyl acetate and isobutyl methoxypyrazine were applied in saline by prolonged superfusion. At 500 nanomolar they elicited periodic bursts of current transients arising from cellular action potentials. The response was rapidly, fully and reversibly blocked by 50 microM amiloride added to the odorant solution. With 10 microM amiloride, the response to odorants was only partially abolished. 2) Whole-cell mode: Following breakage of the patch, the odorant response was lost within 5 to 15 min. Prior to this, odorants evoked a series of slow transient depolarizations (0.1/sec, 45 mV peak to peak) which reached threshold and thus elicited the periodic discharge of action potentials. These slow depolarizing waves were reversibly blocked by amiloride, which stabilized the membrane voltage between -80 and -90 mV. We conclude that amiloride inhibits chemosensory transduction of olfactory receptor cells, probably by blocking inward current pathways which open in response to odorants.

Calcium and olfactory transduction

1. Inorganic cations, organic calcium antagonists, and calmodulin antagonists were applied to olfactory epithelia of frogs (Rana pipiens) while recording electroolfactogram (EOG) responses. 2. Inorganic cations inhibited EOGs in a rank order, reflecting their calcium channel blocking potency: La3+ greater than Zn2+ greater than Cd2+ greater than Al3+ greater than Ca2+ greater than Sr2+ greater than Co2+ greater than Ba2+ greater than Mg2+. Barium ion significantly enhanced EOGs immediately following application. 3. Diltiazem and verapamil produced dose-dependent EOG inhibition. 4. Calmodulin antagonists inhibited EOGs without correlation to their anti-calmodulin potency.

Ion transport across the frog olfactory mucosa: the basal and odorant-stimulated states

The Ussing method was adapted to study the basal electrolyte transfer as well as the events that occur upon odorant stimulation in frog olfactory mucosa. The unstimulated short-circuit current was due mainly to a furosemide-sensitive ion transport system on the apical side of the olfactory mucosa. This current was not amiloride sensitive. The current-voltage relationship of the unstimulated state was linear. That of the odorant-evoked current was non-linear and amiloride-sensitive. Ouabain caused collapse of both the unstimulated and odorant-stimulated short-circuit current. In this case, voltage-clamping the tissue to non-zero values restored the odorant-evoked current with polarity depending on that of the clamping voltage. This suggested that the direction of the current is determined by that of the sodium electrochemical potential difference. Our results indicate that the unstimulated short-circuit current occurs through an apical sodium cotransport system, while the odorant-evoked current is due to odorant-activated, passive sodium channels that are amiloride sensitive.

Taste receptors in crayfish: recording of single nicotinamide-activated channels

The patch clamp technique was adapted to chemoreceptive sensory cells in the claw of Austropotamobius torrentium. In cell-attached and in outside-out patches, concentrations of nicotinamide from 10(-6) mol/liter to 5 X 10(-5) mol/liter activated openings of single channels located in the cell body of the sense cell. The characteristics of these single channel openings were similar to those of neurotransmitter activated channels: the I/V curve was approximately linear, showing a reversal potential of about +15 mV and a conductance of about 33 pS. The mean open time was 1-2 ms. A similar concentration dependence was found for the activation of single channel openings as shown previously for the action potential discharge elicited by nicotinamide at the same preparation.

Voltage- and current-clamp recordings of the receptor potential in olfactory receptor cells in situ

Intracellular recordings obtained using the whole cell configuration of the patch recording technique show that isolated somas of olfactory receptor cells are electrogenic, producing fast overshooting action potentials when depolarized. In situ, these cells produce graded receptor potentials and action potentials when stimulated chemically. The receptor potential can be voltage clamped for analysis of the ionic basis of sensory transduction in olfaction.

Dissociation of frog olfactory epithelium

We report a method for producing cell suspensions from frog olfactory epithelium. The tissue is incubated for 45 min at room temperature in a solution which causes dissociation of the epithelium. The solution is an isotonic saline buffered to maintain a pH of 10.3 and a free Ca2+ ion concentration of 10(-6) M. The method uses no degradative enzymes, except for a brief DNase treatment. The resulting cell suspension contains single olfactory receptor neurons, sustentacular cells, glandular cells, and respiratory epithelial cells. The cells are viable as judged by vital staining, ciliary motility, and synthesis of RNA. Some types of cells lose their normal columnar shapes and become rounded in the suspension.

Intracellular recordings from salamander olfactory receptor cells

Intracellular recordings were obtained from salamander olfactory receptor cells. The occurrence of an intracellular spike in response to the antidromic stimulation of the olfactory fibers was considered as a physiological criterion of a neuronal impalement. The mean resting potential was -56 +/- 9 mV (mean +/- S.D.; n = 70). Fifty-two cells presented a spontaneous spike activity lower than 2 impulses/s. Appropriate olfactory stimulation generally evoked a slow and graded decrease (up to 28 mV) of the intracellular potential. The input resistance of the cell decreased markedly during the response. The slow potential change induced a repetitive firing. Increasing the intensity of the olfactory stimulation increased the instantaneous frequency of firing (up to 25 s-1) and reduced the spike amplitude. The spikes presented an inflexion in the rising phase indicating a two-stage depolarization. With the strongest intensities of stimulation the impulse activity was stopped during the repolarizing phase of the cell response when the membrane potential was still appreciably depolarized.

Dissociation of frog olfactory epithelium with N-ethylmaleimide

Treatment of frog olfactory epithelium with 8 mM N-ethylmaleimide for 2 min results in extensive dissociation of the epithelium. The resulting cell suspension contains single olfactory receptor neurons, sustentacular cells, respiratory epithelial cells, and cells of Bowman's glands. The cells in suspension exhibit the same morphologies seen in histological sections of intact epithelium.

Coacervate-like membrane structures and olfactory transduction

Current theories concerning the olfactory transduction process are discussed. A hypothesis is formulated, according to which the olfactory receptor membrane contains regions where it has the structure of a lipid-protein coacervate. Such structures may well occur in living cells. Such a membrane would have the ability to change its permeability in response to adorants and a sensitivity comparable to that of the sense of smell. The model also explains the fact that different receptor cells have different sensitivity patterns towards odorants. The model is consistent with the results of experiments that seek to establish the locus of odorant action.

The chemistry of olfactory reception: stimulus-specific protection from sulfhydryl reagent inhibition

The group-specific protein reagent, N-ethylmaleimide, irreversibly blocks the electrical response of the olfactory receptor organ of the frog to odorous stimuli. If the odorous substance, ethyl n-butyrate, in concentrations high enough to saturate the receptor system, is present in the nasal cavity before and during a brief exposure to N-ethylmaleimide, the nose, after a wash and a recovery period, responds in nearly normal fashion to vapors of ethyl n-butyrate. Responses to other odorous substances, except those closely related to ethyl n-butyrate, are abolished. We propose that we can use this protection technique to identify the properties of the various receptor sites in the nose, and possibly to characterize the receptor substances.